The present invention relates to ophthalmic optical elements incorporating a image deflecting/magnifying holographic element and, more particularly, to the use of such ophthalmic optical elements positioned within the eye, in intimate contact with the eye, or in front of the eye, for correction of vision defects, central field loss, such as macular degeneration, in particular. The invention further relates to a system for determining an optimal angle to which an image is to be deflected by corrective optics for enabling optimal vision in central vision loss conditions.
As shown in FIG. 1, a normal eye 10 includes a cornea 12, an aqueous solution called the aqueous humor 14 behind cornea 12, an iris 16, a natural lens 18, ciliary sulcus 20, retina 22, macula 24 at the center of the retina, and fovea 26 at the center of macula 24. The cornea 12 and lens 18 cause an image 30 to form at fovea 26. Fovea 26 is a circular zone approximately 0.2-0.5 mm.sup.2 in area. The image 30 formed at fovea 26 corresponds to a locus of fixation for providing acute vision. This locus of fixation helps to coordinate voluntary and involuntary head and eye movements required for daily activities, such as reading, driving, and dressing. Peripheral images are located around this locus of fixation.
A common cause of blindness in adults is macular degeneration. This retinal disease involves damage to the fovea so that the fovea is unable to process images. The blind spot is located at the center of a patient's visual field. The patient is thus unable to read, drive, or perform other tasks that require the brain to reference the locus of fixation.
In most patients, even in those with advanced macular degeneration, the macula is not completely damaged, but retains healthy areas. However, the loss of the locus of fixation caused by the central blind zone leads to severe visual impairment and often to legal blindness, defined as visual acuity of 20/200 or less. The number of patients diagnosed with such severe visual impairment in the United States alone exceeds 2 million.
Intraocular lens implants have been devised to replace the natural lens of the eye and restore sight to damaged or diseased eyes. For example, compound intraocular lenses that combine different optical elements have been proposed. In such proposals, a diffractive/refractive lens implant includes a diffractive lens profile covering about half the effective lens area. Such a configuration allows about half of the incident light from distant objects and half of the incident light from near objects to enter the eye. Such a compound optic provides an ability to form on the retina a focused image of both distant objects and near objects.
Although both images are formed on the fovea, the brightness of the image in each case is reduced by about 50%, or the ratio of the light intensity assigned to each image. In certain cases, such a lens can be used to treat macular degeneration by providing sufficient image magnification so as to project the image over a retinal area more than that damaged by macular degeneration. Such an approach, however, does not shift the image to healthy portions of the retina.
Similar multifocal intraocular lenses incorporating two refractive zones also have been disclosed. For example, the use of a pair of bifocal intraocular lenses has been disclosed in which each of the pair of bifocal intraocular lenses incorporates a refractive element and a diffractive element. One of the lenses provides greater image intensity for the image of near objects, while the other lens provides greater image intensity for the image of distant objects. This approach has the advantage that the incident light can be apportioned or split between the two images in a continuous manner between the two lenses. The disadvantage is that the image is processed by two optical elements, each of which introduces its own aberrations and loss of image contrast so that the performance of the compound lens can be worse than either a diffractive or refractive lens.
Intraocular lenses incorporating a single refractive element also have been devised to shift the image from a damaged portion of the retina to a healthy area. In this respect, a prismatic intraocular lens that includes a convex lens portion for focusing light rays and a prism posterior to the convex lens for deflecting light away from the diseased center of the retina to a functional portion. The prismatic intraocular lens restores the central field vision to a patient.
Several considerations arise before such a prismatic intraocular lens can be prescribed for a patient suffering central field loss. For example, means to fixate the intraocular lens in the eye has to be developed in order to ensure that the lens does not rotate or tilt. Such displacements would cause the shifted retinal image to move, perhaps back to a zone which has become nonfunctional due to macular degeneration. It may also damage the eye tissue. In addition, due to the geometry of the eye, it is necessary to minimize the deepness of the prism wedge while remaining deep enough to redirect an image to a desired location on the retina.
Accordingly, U.S. Pat. No. 5,683,457 to Gupta et al. teaches a pair of intraocular lenses for restoring visual function to a patient with central field loss. The pair of intraocular lenses includes a first lens for implantation into a first eye of the patient to provide vision of targets located at a distance greater than five feet from the first eye and a second lens for implantation into a second eye of the patient to provide vision of targets at a distance less than twelve inches from the second eye. The first and second lenses each include a prismatic wedge for shifting retinal images to a functional portion of a retina of each respective first and second eye.
Similarly, U.S. Pat. No. 4,581,031 to Koziol discloses an intraocular lens including a convex portion and a prismatic portion for use with patients having macular degeneration of the retina. The lens is to be implanted in the eye, after removal of the natural lens, to direct rays of light away from the diseased center of the retina and focus them on a non-affected area of the retina, thereby improving sight. The lens includes a lens portion having a first portion including means for focusing light entering the eye and a second portion including prism means for deflecting light entering the eye away from the center of the retina. The lens further includes means, coupled to the lens portion, for supporting the lens portion in the eye.
The disadvantages of the teachings by Gupta et al. and Koziol are that the deepness and weight of the lenses employed cause inaccurate placement and positional instability resulting in impaired vision and, in severe cases, damage to surrounding eye tissue and post operative need to remove, reposition or change the lens.
It will be appreciated that an equivalent device can be configured as a contact lens. However, the deepness of the prismatic lens will not allow proper eye motion or response to blinking.
There is thus a widely recognized need for, and it would be highly advantageous to have, a holographic ophthalmic element which is thin and light weight in nature, which can be used inter- or intra-ocularly in cases of central field loss, such as macular degeneration, and which avoids the limitations of the prior art described herein.